The present invention relates to a receiving circuit of a wireless device such as satellite communications terminal.
Recently, owing to the development of satellite communications and cellular telephones, interference signals are increasing for reception of a wireless device.
Besides, in the personal computers and other household appliances, there are many devices using clocks of hundreds of megahertz, and a clock noise from them is also an interference for the receivers.
Interference waves outside of the band of desired signal can be removed by using filter characteristics. However, a filter cannot remove an impulse noise in the band of desired signal. In the present environments of electromagnetic waves, such a noise is occurring more and more frequently. Ignition noises of engine, or contact noises of a railcar and a cable is a momentary wide-band noise. Recently, as two-way communications systems using low earth orbit satellites, various systems are introduced, including Orbcomm, Globalstar and Iridium. Everywhere in the terminal bands used in such communications systems, there are sources of impulse noises of such frequency components.
In the case of Orbcomm, for example, a very high frequency (VHF) band is used for the downlink. Therefore, if there is such an impulse noise or its harmonic component in the VHF band, it gets into the band of the receiving terminal. As a result, such an impulse noise has a serious effect on the downlink reception from the satellite.
That is, an impulse noise of high level or its harmonic component becomes a serious interference signal when getting into the band of downlink from the satellite.
Suppose an interference signal of a larger electric power than a desired signal is present in the receiving band of the desired signal for a long period. At this time, as long as the interference signal exists, the desired signal is masked by the interference signal. It is therefore impossible to pick up the desired signal, and the reception-disabled state continues.
The interference signal existing for a long time at a specific frequency is mostly permitted and generated for a specific purpose. Therefore, the district under the influence is limited. In such a case, the effect can be avoided by using the receiver outside of the specific district under the influence of the interference signal.
However, an impulse noise with a shorter pulse width than a symbol period of the data stream in the desired signal may be present everywhere as mentioned above. Therefore, the receiver is required to have a circuit for rejecting an interference by such an impulse noise. Referring now to the drawing, a conventional impulse noise rejection circuit is explained below.
FIG. 4 is a structural diagram of a conventional impulse noise rejection circuit.
In FIG. 4, the impulse noise rejection circuit comprises a switch 32 for cutting off the signal line, an amplifier 33 for amplifying the impulse noise, a detector 34 for detecting the impulse noise, a control signal generator 35 for opening and closing the signal line, and a narrow band pass filter 36.
In the conventional impulse noise rejection circuit having such a configuration, the operation is explained below.
In FIG. 4, the impulse noise getting into the receiving circuit enters the input signal line 31 and impulse noise amplifier 33. In the amplifier 33, the impulse noise is amplified to a level so as to be detected by the detector 34. In the detector 34, the envelope of the impulse is taken out by envelope detection, and is passed through two low pass filters differing in the time constant. If a signal having a steep rise such as an impulse noise is fed into the detector, a potential difference occurs in the outputs of the two low pass filters. This potential difference is detected in the controller 35. When the controller 35 detects a potential difference larger than a specified value, the controller 35 generates a signal for releasing the switch 32. Thus, by the input of impulse noise, the input signal line 31 is cut off. Therefore, the impulse noise is not transmitted to the narrow band pass filter 36. In this manner, effects of impulse noise on reception can be suppressed.
Such a conventional structure, however, requires many components and is large in the circuitry.
It is hence an object of the invention to solve the problems of the prior art and present an impulse noise rejection circuit smaller in the circuitry and reduced in the number of parts.
To solve the problems, the invention employs a limiter amplifier for equalizing the level between the impulse noise and ordinary signal. Thus, an impulse noise rejection circuit smaller in the circuitry and reduced in the number of components is presented.
Further, the invention employs an automatic gain control amplifier for equalizing the level between the impulse noise and ordinary signal. In this structure, too, an impulse noise rejection circuit smaller in the circuitry and reduced in the number of parts is presented.